wcmo_registration_fields.php

<?php
/** 
 * Add custom field to registration form
 */
function my_prefix_registration_fields( $fields ) {
  // Add a text field
  $fields['new_field'] = array(
    'label'	=> __( 'New Field', 'wcmo' ),
    'type'	=> 'text',
    'priority'	=> 250,
    'add_to_profile' => true // Select this if you wish to display this field in user profiles
  );
  // Add a select field
  $fields['new_select_field'] = array(
    'label'	=> __( 'New Select Field', 'wcmo' ),
    'type'	=> 'select',
    'priority'	=> 300,
    'options'	=> array(
	    'option-1'	=> 'Option 1',
	    'option-2'	=> 'Option 2'
    ),
    'add_to_profile' => true // Select this if you wish to display this field in user profiles
  );
  return $fields;
}
add_filter( 'wcmo_registration_fields', 'my_prefix_registration_fields' );